NAIROBI  Photographs taken from space can track deforestation in developing nations and could be used along with cash incentives to safeguard trees and combat global warming, experts say.

Some developing nations, such as Papua New Guinea and Bolivia, want Nov. 6-17 U.N. climate talks in Nairobi to work out ways to reward farmers who slow deforestation, a top source of greenhouse gases behind burning fossil fuels.

Experts said satellite photograph records can help overcome one of the big stumbling blocks -- finding a historical benchmark of forest size against which to give aid to those who slow the rate of destructive logging.

"Since the early 1990s, changes in forest area can be measured from space with confidence," said Frederic Achard, of the U.N.-backed Global Terrestrial Observing System. Pictures from space can back up ground observations.

But experts said there were problems such as gaps in the satellite records -- clouds often mar views of tropical rainforests such as the Amazon -- or judging when forests are being thinned rather than fully cleared.

"It's quite challenging to get good, cloud-free data," said Thomas Hausler, of German company GAF AG which provides satellite data analysis.

"We don't yet have the perfect satellite," said Olivier Arino of the European Space Agency. New generation of satellites from 2012 would improve coverage.

TRADE CREDITS

Most efforts to curb global warming centre on reining in emissions from burning fossil fuels in power plants, factories and cars in industrial nations. Involving forests could add a vast new area, perhaps even a market of tradeable credits.

"If a country is making efforts to preserve forests it should be rewarded," said Gisela Ulloa, the coordinator of Bolivia's National Clean Development Office.

Trees soak up carbon dioxide, the main gas blamed for global warming, as they grow. They release it when they die and rot. Most scientists say a build-up of greenhouse gases could cause more floods, heatwaves, storms, and rising sea levels.

Developing countries say a poor farmer will often sell trees to loggers rather than preserve them because of a lack of incentives to keep them standing.

Forests are home to half the species living on land and a key source of food, building materials and medicines worth billions of dollars a year.

A net 7.3 million hectares (18.04 million acres) of forests -- the size of Panama or Sierra Leone -- was lost each year from 2000-2005, according to U.N. data.

Hausler said that full, comparable, satellite maps had been compiled for Bolivia for 1990, 2000 and 2005 from U.S. and European data.

He said it was harder, for instance, to find data for Cameroon. "Here it's a bit more difficult because we have to deal with a lot of cloud coming from the Atlantic," he said.